1. Technical Field
The present invention relates to a method and the like for receiving a carrier wave of a navigation message that is modulated according to the BPSK (Binary Phase Shift Keying) modulation scheme.
2. Related Art
GPS (Global Positioning System) is widely known as a positioning system that uses a positioning signal, and is used in a receiver that is included in a cellular phone, a car navigation device, and the like. In GPS, the positions of a plurality of GPS satellites and a pseudo distance or the like from the respective GPS satellites to a receiver are obtained using the time measured by a GPS receiver, and finally, a position calculation is performed.
A GPS satellite transmits information regarding a satellite orbit, time, and the like at a bit rate of 50 bps (bits per second) by embedding the information in a navigation message. One navigation message includes one master frame, and one master frame includes 25 frames made up of 1500 bits. In order to acquire all navigation messages, it is necessary to receive the 25 frames that constitute the master frame, which requires 12.5 minutes.
The reception conditions of the GPS satellite signal change depending on the position or the like of a receiver. In a weak field environment, since the signal strength of a received signal is weak, it is difficult to accurately detect a change (bit transition) in the bit value of the navigation message, and there is a possibility that the demodulation of the received signal results in error. JP-T-2008-541629 discloses a method for determining whether the result of the demodulation of a received signal has an error or not.
JP-T-2008-541629 discloses error detection which involves performing checking according to a predetermined computation formula using a 6-bit error detection bit stored at the end of each of the words that are included in the respective subframes of a navigation message.
On the other hand, the GPS satellite signal is subjected to BPSK modulation according to a bit value of the navigation message. Thus, if one demodulation error occurs when a received signal of the GPS satellite signal is demodulated to received data, a problem in which all items of the subsequently demodulated data are in error may occur. Such a problem is noticeable in an environment (weak field environment) where the received signal is a weak electric field signal.
In this case, the checking result of the error detection is highly likely to be “NG”. However, it is not efficient to discard received data which have not been correctly demodulated all the time due to one demodulation error. For example, in a weak field environment or the like, if demodulated data is repeatedly considered as “NG”, it takes a significant amount of time for collection of parameters necessary for satellite orbit computations, which causes a problem in that the time to first fix (TTFF) increases.